Fibrous structures, such as paper webs, are produced by a variety of processes. For example, paper webs may be produced according to commonly-assigned U.S. Pat. Nos. 5,556,509, issued Sep. 17, 1996 to Trokhan et al.; 5,580,423, issued Dec. 3, 1996 to Ampulski et al.; 5,609,725, issued Mar. 11, 1997 to Phan; 5,629,052, issued May 13, 1997 to Trokhan et al.; 5,637,194, issued Jun. 10, 1997 to Ampulski et al.; and 5,674,663, issued Oct. 7, 1997 to McFarland et al., the disclosures of which are incorporated herein by reference. Paper webs may also be made using through-air drying processes as described in commonly-assigned U.S. Pat. Nos. 4,514,345, issued Apr. 30, 1985 to Johnson et al.; 4,528,239, issued Jul. 9, 1985 to Trokhan; 4,529,480, issued Jul. 16, 1985 to Trokhan; 4,637,859, issued Jan. 20, 1987 to Trokhan; and 5,334,289, issued Aug. 2, 1994 to Trokhan et al. The disclosures of the foregoing patents are incorporated herein by reference.
Removal of water from the paper in the course of paper-making processes typically involves several steps. Initially, an aqueous dispersion of fibers typically contains more than 99% water and less than 1% papermaking fibers. Almost 99% of this water is removed mechanically, yielding a fiber-consistency of about 20%. Then, pressing and/or thermal operations, and/or through-air-drying, or any combination thereof, typically remove less than about 1% of the water, increasing the fiber-consistency of the web to about 60%. Finally, the remaining water is removed in the final drying operation (typically using a drying cylinder), thereby increasing the fiber-consistency of the web to about 95%.
Because of such a great amount of water needed to be removed, water removal is one of the most energy-intensive unit operations in industrial paper-making processes. According to one study, paper-making is the leading industry in total energy consumption for drying, using more than 3.75.times.10.sup.14 BTU in 1985 (Salama et al., Competitive Position Of Natural Gas: Industrial Solids Drying, Energy and Environmental Analysis, Inc., 1987). Therefore, more efficient methods of water removal in the paper-making processes may provide significant benefits for the paper-making industry, such as increased machine capacity and reduced operational costs.
It is known in the papermaking arts to use steady-flow impingement gas and cylinder dryers to dry a paper web. (See, for example, Polat et al., Drying Of Pulp And Paper, Handbook Of Industrial Drying, 1987, pp. 643-82). Typically, impingement hoods are used together with Yankee cylinder dryers for tissue products. In webs having relatively low basis weights of about 8-11 pounds per 3000 square feet, water is removed in about 0.5 seconds. This corresponds to an evaporation rate of about 42 pounds per hour per square feet, with about 75% of the total evaporation being performed by the impingement hood. The drying rates of paper products having relatively heavier basis weights are considerably slower. For example, newsprint, having a basis weight of about 30 pounds per 3000 square feet, has the evaporation rate of about 5 pounds per hour per square feet on the cylinder dryers. See, for example, P. Enkvist et al., The Valmet High Velocity and Temperature Yankee Hood on Tissue Machines, presented at Valmet Technology Days '97, Jun. 12-13, 1997, at Oshkosh, Wis., USA.
It is also known to use a sonic energy, such as that generated by steam jet whistles, to facilitate removal of water from various products, including paper. U.S. Pat. No. 3,668,785, issued to Rodwin on Jun. 13, 1972, teaches sonic drying and impingement flow drying in combination for drying a paper web. U.S. Pat. No. 3,694,926, issued to Rodwin et al. on Oct. 3, 1972, teaches a paper dryer having a sonic drying section through which the web is passed and subjected to high intensity noise from grouped noise generators, to dislocate moisture from the web. U.S. Pat. No. 3,750,306, issued to Rodwin et al. on Aug. 7, 1973, teaches sonic drying of webs and rolls, involving steam jet whistles spaced along trough-like reflectors and low pressure secondary air to sweep displaced moisture clear of the traveling web.
The foregoing teachings provide a means for generating sonic/acoustic energy and a separate means for generating steady-flow impingement/wiping air. Generating the acoustic energy in accordance with the prior art by such means as noise generators, steam whistles, and the like requires very powerful acoustic sources and leads to a significant power consumption. It is well known in the art that the efficiency of the conventional noise generators, such as sirens, horns, steam whistles, and the like typically do not exceed 10-25%. An additional equipment, such as auxiliary compressors to pressurize air, and amplifiers to generate the desired sound pressure, may also be necessary to reach a desired drying effect.
Now, it has been found that impingement of a paper web with air or gas having oscillatory flow-reversing movement, as opposed to a steady-flow impingement of the prior art, may provide significant benefits, including higher drying/dewatering rates and energy savings. It is believed that an oscillatory flow-reversing impingement air or gas having relatively low frequencies is an effective means for increasing, relative to the prior art, heat and mass transfer rates in papermaking processes.
Pulse combustion technology is a known and viable commercial method of enhancing heat and mass transfer in thermal processes. Commercial applications include industrial and home heating systems, boilers, coal gassification, spray drying, and hazardous waste incineration. For example, the following U.S. Patents disclose several industrial applications of pulse combustion: 5,059,404, issued Oct. 22, 1991 to Mansour et al.; 5,133,297, issued Jul. 28, 1992 to Mansour; 5,197,399, issued Mar. 30, 1993 to Mansour; 5,205,728, issued Apr. 27, 1993 to Mansour; 5,211,704, issued May 18, 1993 to Mansour; 5,255,634, issued Oct. 26, 1993 to Mansour; 5,306,481, issued Apr. 26, 1994 to Mansour et al.; 5,353,721, issued Oct. 11, 1994 to Mansour et al.; and 5,366,371, issued Nov. 22, 1994 to Mansour et al., the disclosures of which patents are incorporated by reference herein for the purpose of describing pulse combustion. An article entitled "Pulse Combustion: Impinging Jet Heat Transfer Enhancement" by P. A. Eibeck et al, and published in Combustion Science and Technology, 1993, Vol. 94, pp. 147-165, describes a method of convective heat transfer enhancement, involving the use of pulse combustor to generate a transient jet that impinges on a flat plate. The article reports enhancements in convective heat transfer of a factor of up to 2.5 compared to a steady-flow impingement.
The applicant believes that the oscillatory flow-reversing impingement can also provide significant increase in heat and mass transfer in web-dewatering and/or drying processes, relative to the prior art dewatering and/or drying processes. In particular, it is believed that the oscillatory flow-reversing impingement can provide significant benefits with respect to increasing paper machine rates, and/or reducing air flow needs for drying a web, thereby decreasing size of the equipment and capital costs of web-drying/dewatering operations and--consequently--an entire papermaking process. In addition, it is believed that the oscillatory flow-reversing impingement enables one to achieve a substantially uniform drying of the differential-density webs produced by the current assignee and referred to herein above. It is now also believed that the oscillatory flow-reversing impingement may be successfully applied to dewatering and/or drying of fibrous webs, alone or in combination with other water-removing processes, such as through-air drying, steady-flow impingement drying, and drying-cylinder drying.
To be able to effectively remove water from the web, the oscillatory flow-reversing air or gas should in most cases act upon the web in a substantially uniform manner, especially across the web's width (i. e., in a cross-machine direction). Alternatively, one might desire to differentiate, in a particular pre-determined manner, the application of the oscillatory impingement gas across the width of the web, thereby controlling relative moisture content and/or drying rates of differential regions of the web. In either instance, the control over the distribution of the oscillatory flow-reversing air or gas throughout the surface of the web, and particularly in the cross-machine-direction, is crucial to the effectiveness of the process of removing water from the web.
Paper webs produced on modern day's industrial-scale paper machines have width of about from 100 to 400 inches, and travel at linear velocities of up to 7,000 feet per minute. Such a width, coupled with a high-speed movement of the web creates certain difficulties of controlling (presumably uniform) distribution of the oscillatory gas throughout the surface of the web. Existing apparatuses for generating oscillatory flow-reversing air or gas, such as, for example, pulse combustors, are not well adapted, if at all, to generate a required substantially uniform oscillatory field of the flow-reversing air or gas across a relatively large area.
Accordingly, it is an object of the present invention to provide a process and an apparatus for removing water from fibrous webs, using the oscillatory flow-reversing impingement gas. It is another object of the present invention to provide a gas-distributing system allowing one to effectively control the distribution of the oscillatory flow-reversing air or gas throughout the surface of the web. It is still another object of the present invention to provide a gas-distributing system that creates a substantially uniform application of the oscillatory flow-reversing air or gas onto the web.